The present invention relates to a pressure regulation device for a vehicle safety air bag which is provided to protect a passenger of a car or other vehicle from a secondary collision. In an air bag of this type a bulbous-shaped envelope is folded and stored at the center of the steering wheel and is rapidly inflated by a gas generated virtually instantaneously by an inflator at the moment of a collision in order to prevent the driver, who is thrown forward by inertial force, from hitting the steering wheel directly, to absorb and damp the energy imposed on the driver by the inertial force, and to spread the load of the impact over the driver's torso.
The maximum internal pressure of the air bag main envelope is typically 1.0 kg/cm.sup.2 when it is inflated, and its normal pressure is 0.7 kg/cm.sup.2. Therefore, the air bag envelope must be fabricated so as to withstand these pressures, but if the pressure is too high, a high repelling or rebound force will develop and will propel the driver back away from the air bag, thereby possibly causing whiplash injury to the driver. It is known, therefore, to provide a pressure-relief device in order to keep the internal pressure from exceeding a predetermined level. One such device is simply a vent orifice that allows gas to escape when the driver strikes the inflated envelope and the internal pressure rises.
In particular, the conventional pressure-relief vent hole structures are of the type wherein a normally-open vent hole is disposed in the envelope and of the type wherein a film of a synthetic resin or the like is disposed in the vent hole so that the film is broken by a gas pressure above a predetermined pressure. In the case of the open vent hole, if the size of the hole is determined based on a low external or ambient temperature, the gas pressure will become excessive when the temperature is high, and if the size of the hole is determined based on a high ambient pressure, the gas pressure will be insufficient when the temperature is low. In the case of vent holes with a rupturable film, when the film is broken at pressures above a predetermined level, the fragments of the film scatter. Therefore, the open area of the vent hole remains substantially the same irrespective of the gas pressure, and the gas pressure, which varies with the ambient temperature, cannot be regulated. Thus, there is in both types of vent devices variation in the damping property when the driver hits the bag, as a function of ambient temperature.